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The GM 21" wheels add rotational mass to the drivetrain, resulting in more torque needed to produce the same rotational speed as a 20" or 19" wheel, which will make the stock car slower with that wheel package. As for the height of the wheels and the effect on speed....a taller rim will need a smaller sidewall to keep the same rotation length and vice-versa. GM puts factory wheels, of whichever height, with Pirelli P-zero's on the car, and the computers are programmed for that wheel and tire set. Any change in rim size or tire sidewall without countering in the other parameter will change the overall height of the wheel and tire set, consequently changing the speedo readings unless the computer is reprogrammed accordingly. However, that won't matter at the strip, or in a head's up race. That said, the larger rim size can cause a slower time in the 1/4 vs. the same set-up power-train using a smaller diameter rim, but that extra time can easily be overcome by a better driver, or a higher power, specifically torque, output.
Hope that helps!
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